In the past, considerable experimentation has been had with respect to reducing the friction between the wire core and conduit of push-pull cables. To this end the cable core has been coated with polymeric material such as Teflon, and the conduit has been lined with nylon or Teflon. An example of such construction is disclosed in U.S. Pat. No. 3,176,538, in which the twisted wire core has a plastic covering of Teflon, and the conduit has an inner layer of nylon. The patent states that other suitable plastics may be used but the core coating of Teflon and the inner conduit layer of nylon are preferred because of their self-lubricating characteristics.
While the Teflon coated core and the nylon or plastic lined conduit of U.S. Pat. No. 3,176,538 have improved efficiency as compared with prior push-pull cables having a wire core and a helical wire casing, the patented construction still had a high noise transmission level and considerable friction, because of the limited amount of self-lubrication inherent in the plastic materials of the core coating and conduit lining. Moreover, it is difficult continuously to form the inner plastic lining of the conduit with a smooth inner surface.